Multiplexing is a method by which multiple channels of analog or digital data are combined on placed into a single shared media at the input end of the media. The media may be any communication media, such as, for example, an optical fiber. De-multiplexing is a method by which multiplexed signals are recovered from the shared media and separated into individual channels at the receiving end of the media. Optical multiplexing systems bring distinctive advantages over traditional non-optical systems. These advantages include significant bandwidth increase and higher data transmission rates. Multiplexing techniques in optical communications include, among others, time division multiplexing (TDM) wavelength division multiplexing (WDM), dense wavelength division multiplexing (DWDM), orbital angular momentum (OAM) multiplexing, spatial domain (or space division) multiplexing (SDM), time division multiplexing (TDM), and polarization division multiplexing (PDM).
TDM is a method for combining two or more separate streams of data, which may be digital data or analog data, for communication over a common channel, which may be for example a single optical fiber. In TDM, the incoming separate streams of data are divided into segments or packets which may be of equal or predetermined length. The packets may be encoded, encrypted, or otherwise manipulated for data integrity and security reasons. The packets containing data from the incoming separate streams of data are interleaved in time by the TDM multiplexer, resulting in a multiplexed data stream that contains packets of data from the incoming separate streams of data which are interleaved in time. After multiplexing, the multiplexed data signal is transmitted over a shared communication medium, such as an optical fiber, where it is received by a receiver. The multiplexed data signal is demultiplexed on the receive, or output end of the shared communication medium. The packets for each incoming separate streams of data are recovered from the multiplexed data signal and reassembled into their original format to recreate each original incoming separate streams of data.
WDM, illustrated in FIG. 1, allows simultaneous propagation of independent channels of different optical wavelengths (or, in other words, colors) into a single optical fiber. Those channels are multiplexed and launched into the optical fibers at the input end. On the receiving end of the fiber, a wavelength division de-multiplexer separates the signals based on each individual channel wavelength. Optical filters, such as fiber-based Bragg Gratings, are typically used for this purpose in conjunction with photo-detectors to demultiplex WDM signals into individual communication channels. A fiber Bragg grating (FBG) is a type of distributed Bragg reflector constructed in a short segment of optical fiber that reflects particular wavelengths of light and transmits all others. This is achieved by creating a periodic variation in the refractive index of the fiber core, which generates a wavelength-specific dielectric mirror. A fiber Bragg grating can therefore be used as an inline optical filter to block certain wavelengths, or as a wavelength-specific reflector for use as a WDM demultiplexer.
In OAM multiplexing, two different orthogonal electromagnetic waves are multiplexed onto a single optical communication channel using two independent and different orbital angular momentums of the same azimuthal index. This momentum can be either clockwise (CW) or counter clockwise (CCW). Based on the azimuthal index, OAM can be detected using, among other techniques, a ridge-based segmented circular detector, such as the one shown in FIG. 15A. The structure and method for using a ridge-based segmented circular detector is described in U.S. Pat. No. 8,396,371, which is hereby incorporated by reference in its entirety.
SDM utilizes a MIMO configuration to increase the data capacity of optical fibers. The data carrying capacity of a standard optical fiber increases as helically propagating non-meridional SDM channels allow spatially separated channels to reuse optical frequencies within an optical fiber. SDM has been successfully tested up to several kilometers. It allows multiple channels of the same optical wavelength to propagate inside a single multimode carrier optical fiber (which may be, for example, 62.5/125 μm). Concentric donut shape rings, one ring for each independent channel, are generated at the output end of the system due to helical propagation of light while traversing the length of the fiber. A spatial domain de-multiplexer having photodetectors spatially arranged so that at least one photodetector, or a plurality of photodetectors, is individually illuminated by each of the independent concentric rings is used to separate the individual SDM output channels. Thus, each ring emitted from the receiving, or output, end of the optical fiber illuminates a specific photodetector, or plurality of photodetectors, for converting each ring into an independent channel of electrical data. As shown in FIG. 2, a typical SDM system includes a plurality of optical sources, such as single-mode pigtail laser sources of a given wavelength, a beam combiner module (BCM) or spatial multiplexer, a standard step index multimode carrier fiber, a beam separator module (BSM) or spatial de-multiplexer and photo detectors to detect the different channels. In use, light from multiple single-mode pigtail laser sources of the same wavelength is launched into a carrier multimode step index fiber at an angle specific to each source. The launching angles determine the output angles of the light at the output end of the carrier fiber. Each spatially separated optical channel launched into the fiber follows a separate helical trajectory while traversing the length of the carrier fiber thereby allowing multiple spatially separated optical communication channels of the same wavelength to exist simultaneously in the fiber; in other words, allowing spatial reuse of optical frequencies. Light is launched from multiple single-mode pigtail optical sources, such as lasers, at different angles (with respect to the longitudinal axis of the carrier fiber) into the carrier fiber. Free space, few mode and multimode fiber based laser sources have also been successfully used. Due to helical propagation, distinct concentric donut-shaped rings, one ring for each independent channel, with no discernible cross talk are produced at the receiving end of the fiber. Each ring is detected for the recovery of data in that specific channel.
These SDM channels can also exhibit OAM thereby adding an extra degree of photon freedom. An SDM system can operate at different wavelengths without changing its radial distribution.